A SharedNotebook is the record of a share of a Notebook N from account A to account B. It is stored in the noteStoreUrl of account A (notebook N). Account B will have a corresponding LinkedNotebook record that points to the SharedNotebook (not the notebook). The LinkedNotebook will be on account B.

Sounds like you're in account B and trying to access the Notebook N. What you want to do is fetch the noteStoreUrl from the LinkedNotebook L and then use that to call authenticateToSharedNotebook. You can fetch the LinkedNotebooks by calling listLinkedNotebooks.